The present invention relates to apparatus and methods for sputtering ionized material from a solid target, or cathode, onto a workpiece with the aid of an inductively coupled plasma.
A number of semiconductor device fabrication procedures include processes in which a material is sputtered from a target onto a workpiece, such as a semiconductor wafer. Material is sputtered from the target, which is appropriately biased, by the impact of ions created in the vicinity of the target. A certain proportion of the sputtered material is ionized by a plasma and the resulting ions are attracted to the wafer.
The wafer is mounted on a support and is biased to a DC potential selected to attract the sputtered, ionized material. Typically, the sputtered material is composed of positive ions and the workpiece is negatively biased.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the primary winding of a transformer. The plasma acts as a single turn secondary winding of the transformer.
In a number of deposition chambers such as a physical vapor deposition chamber, the chamber walls are often formed of a conductive metal such as stainless steel. Because of the conductivity of the chamber walls, it is often necessary to place the RF coils or electrodes within the chamber itself because the conducting chamber walls would block or substantially attenuate the electromagnetic energy radiating from the antenna. As a result, the coil may be directly exposed to the deposition flux and energetic plasma particles. This is a potential source of contamination of the film deposited on the wafer, and therefore may be undesirable in some applications. To protect the coils, shields can be made from nonconducting materials, such as ceramics. However, many deposition processes involve deposition of conductive materials such as aluminum on the electronic device being fabricated. Because the conductive material will coat the ceramic shield, it will soon become conducting, thus again substantially attenuating penetration of electromagnetic radiation into the plasma.
U.S. Pat. No. 5,346,578 describes a system in which a plasma is created for the performance of various types of wafer processing operations, including etching and chemical vapor deposition in a hemispherical quartz vessel surrounded by a similarly shaped exterior induction coil. RF energy is transmitted from the coil through the dome into the chamber to energize the plasma. In the operation of the apparatus discussed in this patent, a reactive gas is introduced into the treatment chamber in order to be ionized by the plasma, the resulting ions being directed to a wafer under the influence of a suitable electric field. It is believed that the apparatus described in this reference is not suitable for the performance of conductive material sputtering processes because sputtered material tends to coat all interior surfaces of a chamber. Hence, the quartz dome would soon become relatively opaque to the RF energy from the coil.
Published European Patent Application 0607797 describes a device for generating a plasma in order to perform low pressure chemical vapor deposition or reactive ion etching operations. The system includes a processing chamber having, at its top, a planar spiral coil producing an electromagnetic field which will be coupled with a plasma within the processing chamber, the coil itself being isolated from the interior of the chamber by a flat dielectric window. The window is associated with a conductive shield which is positioned between the window and the coil. The purpose of the shield is to prevent dielectric material from being sputtered from the window. This publication suggests that the window and coil may, alternatively, be domed or hemispherical.
The material which is to be ionized in order to be deposited on a wafer or to perform etching is introduced into the chamber in the form of a process gas. As in the case of the apparatus described in U.S. Pat. No. 5,346,578, supra, the surface of the dielectric window which communicates with the interior of the chamber is prone to being coated with deposition material. Therefore, it is believed that this chamber is likewise not well suited to sputtering processes.
It is an object of the present invention to provide an inductively coupled plasma processing apparatus which avoids drawbacks possessed by prior art apparatus of this type.
A more specific object of the invention is to produce a more uniform plasma in a processing chamber which contains a metal sputtering target.
Another object of the invention is to protect a dielectric window forming part of the boundary of the processing chamber against deposition of sputtered material.
A further object of the invention is to provide an improved sputtering apparatus having an external coil for inductively coupling energy into the plasma in the processing chamber.
The above and other objects and advantages are achieved, according to the present invention, by an apparatus and method for processing workpieces, which include a chamber having a coil for inductively coupling RF energy through a dielectric window into the chamber to energize a plasma, and a shield positioned between a sputtering target and the dielectric window to reduce or eliminate deposition of sputtered material onto a portion of the dielectric window. In the illustrated embodiment, the window shield is spaced from the dielectric window to define a gap and has at least one opening, which permit RF energy to be coupled through the gap and through the window shield opening to the interior of the chamber. As a consequence, the coil may be positioned exterior to the chamber to simplify construction and operation of the chamber.
In another aspect of the invention, the formation of a plasma having more uniform characteristics is facilitated. For example, by placing the plasma energizing coil outside the chamber, the coil may be readily shaped and positioned to achieve a desired plasma distribution without concern for the interaction between the coil structure and the flow of sputtered material in the chamber. Still further, multiple coils can be used while avoiding the expense and complexity of connecting multiple coils to external RF sources through the walls of the chamber.
In yet another aspect of the invention, the formation of a more uniform sputter deposition material flux may be facilitated. For example, by placing the plasma energizing coil or coils outside the chamber, the sputtering target or targets may be more readily shaped and positioned to achieve a desired deposition flux without concern for the interaction between any internal coil structure and the target. In various illustrative embodiments, the target has an annular ring structure which may improve the uniformity of deposition onto the workpiece.